Tom Cruise -- Honky Tonk Superstar ... Parties with Southern Girls (PHOTOS)

Tom Cruise went hardcore all-American in a Nashville bar -- 'cause nothing says happy birthday, America ... like partying with cute Southern babes in cowboy hats!

Cruise hit a rooftop party at Acme Feed & Seed, and even sprung for the $200 cover to party with the common folk and take in the biggest fireworks show in the country.

We're told Cruise rolled through with minimal security and happily posed for pics with tons of fans. As the night went on, Tommy boy was spotted chatting up some chicks, dancing to country music, drinking champagne and even dipped into the taco and slider station.

It's unclear why he was in town, but everyone got a kick out of seeing him ... even the guy who shook his hand in the bathroom.